Technology for casting methyl methacrylate polymer (PMMA) sheet between belts of stainless steel to obtain smooth surfaces is well known, as represented by U.S. Pat. No. 4,133,861--Kato (1979).
Group transfer polymerization (GTP), as described in several patents including U.S. Pat. Nos. 4,414,372--Farnham, et al. and 4,417,034--Webster, both 1983, and 4,508,880--Webster and 4,524,196--Farnham, et al., both 1985. AB dispersants made by GTP are the subject of U.S. Pat. No.4,656,226--Hutchins, et al. (1987) which suggests use in clear or filled cast acrylic sheet.
In the casting of PMMA sheet, there are phenomena which cause shifting in color, apparently due to competition for attachment sites on the pigment surfaces by additives such as chain transfer agent and sheet release agent. When such additives replace the dispersants from the pigment surfaces, the dispersing effect is diminished because the additives are less efficient as dispersants. With diminished dispersing effect, there is more of a tendency for pigment particles to flocculate, leading to color changes. These effects are aggravated by thermal shock from casting cool syrup on a relatively warm metal belt.